This application claims priority to German application No. 19838553.6, filed Aug. 25, 1998.
The invention relates to a Faraday collector for measuring ion currents in mass spectrometers, having a cup for capturing the ions, an entrance slit and, in particular, a secondary electron diaphragm.
Faraday collectors are used for the precise measurement of ion currents, in particular in the sector field mass spectrometers. Ions produced by an ion source are focused by means of ion optics onto the entrance slit of a mass analyser. The mass analyser consists of at least one magnetic sector field which may be combined with one or more electrostatic sector fields and suitable drift paths. In the simplest version, the mass analyser consists of an entrance slit, a field-free drift path, followed by a magnetic sector field for mass resolution, and a further field-free drift path at the end of which there is a detector having collectors.
The field-free drift paths and the geometry of the magnetic sector fields together form an ion optical system. The dimensions are selected in such a way that ion beams of different mass, which pass through the entrance slit of the spectrometer, are sharply projected onto various positions along the so-called image curve of the mass analyser. In order to measure these mass-resolved ion currents, Faraday collectors are positioned along the image curve. These customarily consist of at least one entrance slit, a secondary electron suppression diaphragm and a collector cup.
Since a plurality of Faraday collectors need to be positioned in parallel along the image curve, there are geometrical constraints on their design dimensions. The maximum width of the collector is limited by the minimum distance between the two adjacent mass-resolved ion currents which need to be measured simultaneously. This distance is typically about 2 to 3 mm, but may also be smaller or larger. The small geometrical size represents a problem for reliable and precise construction of Faraday collectors.
When the ions strike the inside of the collector cup, a wide range of secondary particles is produced, namely secondary electrons, positive secondary ions, negative secondary ions, neutral particles and photons. Further to this, there is also a possibility that, when they xe2x80x9cimpactxe2x80x9d on the collector surface (cup end), the incident particles will be directly reflected and scattered out of the collector cup.
In order for the Faraday collector to measure the charge current of the ions correctly and without distortion, no charged secondary particles and no injected ions should leave the Faraday collector again. If they did, this would vitiate the ion current measurement. This leads to particular requirements in terms of the design and structure of a Faraday collector:
1. the collector cup should be as narrow and deep as possible so that, merely on geometric grounds, the probability of a particle""s escaping is low.
2. The collector should be precisely aligned so that the ions enter the collector cup to the greatest possible depth. This is intended to prevent the sides from being hit when still close to the input opening and secondary particles being created which can leave the collector.
3. The collector cup should be ideally tight.
4. The secondary electron diaphragm should be fitted tightly and precisely in front of the collector cup in order to effectively brake the charged negative secondary particles and return them into the collector cup.
5. The collector cup must be electrically screened against external scattered particles.
6. The internal surface of the collector should consist of a material with low secondary particle yields.
7. The collector must be clean, free of dust and everywhere electrically conductive so that the electric charge can be dissipated quantitatively and no local build-up of charges is created.
Particular demands are made of the mechanical design of the collector. Narrow mechanical tolerances of about 0.05 mm need to be met in a small space. Added to this is the problem that some of the individual parts of the Faraday collector need to be electrically insulated against earth potential (for example the inner collector cup and the secondary electron diaphragm). Typically, insulation resistances of better than 1013 ohms need to be ensured.
The inner collector cups of modern collectors (also referred to as Faraday cups) generally consist of small metal cups which are internally lined in a variety of ways with graphite. Graphite or carbon surfaces are particularly suitable since this material has good electrical surface properties. For example, the collector cups are painted with a kind of graphite paint or graphite powder, or internally lined with thin graphite cladding. This indirect lining of the inner cup with graphite paint causes problems with reproducibility of the surface preparation and durability of the layers. Even after relatively short periods of use, the electrical properties of these layers can change. The lining with thin graphite plates further limits the usable internal width of the collector. The internal graphite plates must be mechanically stable and reproducibly fitted into the collector cup and held. This often represents a particular technical problem.
The object of the present invention is to provide a Faraday collector having improved mechanical properties. The Faraday collector according to the invention is characterized in that the cup has solid graphite walls. Whereas conventional collectors have sheet metal walls lined with graphite, the collector according to the invention has solid graphite walls.
The collector cup is advantageously entirely made of graphite and therefore only has walls of solid graphite.
It is advantageously made of two graphite half-shells connected to one another. They contain a cavity between them which is open on one side for the ions to enter.
The half-shells are held together by clamps which at the same time serve as spacers against an outer frame.
The material of the cup is high-purity solid graphite. The two half-shells have extremely good geometrical stability, fit into one another exactly using a precision groove and form a light-tight labyrinth-like connection. High mechanical stability, high reproducibility of surface properties and optimum use of the limited collector width dictated by the ion optics are achieved. It is not necessary for any additional layers, which would unnecessarily reduce the effective internal width of the collector, to be applied indirectly to the collector. The half-shells are machined from a uniform block of material and therefore have an electrically uniform surface, which is reproducible. Adhesion problems of indirectly applied layers and mechanical problems of holding thin cladding are eliminated.
Other features of the invention can moreover be found in the description and the claims. Illustrative embodiments of the invention will be explained in more detail below with reference to the figures, in which: